The present invention relates generally to methods and apparatuses for evaluating the composition of an oil sample and more particularly to a method and apparatus for evaluating the composition of an oil sample using optical spectroscopy.
As can readily be appreciated, it is very desirable for those persons in the oil industry to be able to evaluate the composition of an oil sample. For example, one may want to know the composition of a crude oil sample since certain components of crude oil, such as asphaltenes and paraffins, negatively affect crude oil quality. In the past, the composition of crude oil samples have typically been determined chemically, e.g., by using various chromatography techniques.
The ability to measure crude oil "instability," that is, the tendency for solid particles to precipitate from their host fluids, has considerable economic value. The solids in question are essentially asphaltenes, high molecular weight polynuclear aromatic hydrocarbons, which have limited solubility in the crude oil despite its being also of hydrocarbon composition. Often, they do not exist in true solution but, instead, in finely divided particulate or colloidal suspension.
Such metastable states are disturbed by lifting and pumping and by comingling with other substances or reagents. For example, it is known that gas lifting, that is, the forcing of oil from its formation by compressed gas, can cause precipitation. Also, carbon dioxide, a polar gas, is more troublesome than, say, methane which is the major component of natural gas.
These precipitates build up and coat and clog well shafts, pipelines and storage vessels. Remedies include chipping, sandblasting and heating, all labor intensive and costing downtime. So-called heavy crudes have higher asphaltene content, but factors affecting instability are often subtle and discovered only empirically.
Polyaromatic structures have characteristic fluorescence. Hence, sampling and analysis on-site in real time could allow field personnel to adjust operating conditions to minimize the effects of this instability.
In U.S. Pat. No. 5,131,398 to Alfano et at., which issued Jul. 21, 1992 and which is incorporated herein by reference, there is disclosed a method and apparatus for distinguishing cancerous tumors and tissue from benign tumors and tissue or normal tissue using native fluorescence. The tissue to be examined is excited with a beam of monochromatic light at 300 nanometers (nm). The intensity of the native fluorescence emitted from tissue is measured at 340 and 440 nm. The ratio of the two intensities is then calculated and used as a basis for determining if the tissue is cancerous as opposed to benign or normal. The invention is based on the discovery that when tissue is excited, for example, with monochromatic light at 300 nm, the native fluorescence spectrum over the region from about 320 nm to 600 nm for cancerous tissue is substantially different from that for tissue that is either benign or normal. The technique is useful in both in vivo and in vitro testing of human as well as animal tissue.